pkgconfig: fix include flags

Vicente Bolearequested to merge
vbolea/vtk-m:correct-pkgconfig-include into master

Our headers are installed in a include namespaced folder named vtk-m-SOVERSION, this is now reflected in this change.

Fixes: #797 (closed)
Backport: release
Backport: release-2.0

Edited by Vicente Bolea

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